This invention relates to an articulated dump truck (ADT) which consists of two vehicle chassis frames mounted on wheels and linked by a steerable articulation joint, with a tipping load container mounted pivotally on the rear chassis while the front chassis carries the cab and prime mover. The articulation joint allows rotation around two axes, namely a first vertical axis and a second longitudinal axis. The rear chassis is mounted on four wheels, two wheels being displaced longitudinally on each side, while the front chassis is mounted on two wheels.
The vehicle is steered by controlling the relative rotation of the chassis frames around the vertical axis at the articulation joint, usually by means of hydraulic rams. The articulated joint also incorporates a large bearing with axis of rotation along the longitudinal axis of either the front or the rear chassis frame in order to decouple the roll of the front and rear chassis frames. This confers advantage in traversing rough terrain by allowing the front and rear chassis frames to maintain better wheel contact with the ground.
Existing designs of ADT provide limited or no roll compliance in the front vehicle suspension, relying mostly on this rotational degree of freedom around the longitudinal axis augmented by tyre compliance, to maintain wheel contact when traversing rough terrain. This approach works well at low speeds. However, at higher speeds single wheel bumps are accompanied by a severe dynamic rolling action of the entire front vehicle body about the longitudinal axis of the articulation joint. Also, due to the high driving position typical of these vehicles, this rolling action subjects the driver to significant lateral accelerations resulting in an uncomfortable ride. Thus the vehicle is effectively limited to low speed operation over rough terrain and this in turn limits productivity.
A suspension system is desirable in order to isolate the chassis frame and hence the driver from road inputs. It also helps to keep good contact between the wheels and the ground thus ensuring good traction at all times. Ideally for good ride the suspension should have a long wheel travel and a low natural frequency. This is achieved by using soft springs. Unsprung mass should also be minimised. Unfortunately, soft springs may result in excessive roll of the vehicle when cornering. Hence there is a trade off between the ride and handling properties of the vehicle. This trade off is particularly acute for ADTs. Inherent with the articulated steering system is the disadvantage that, as the vehicle turns, the outside wheel of the front chassis frame moves closer to the centreline of the load carrying rear chassis frame and consequently bears more of the front axle load. This significantly reduces the roll resistance of the front chassis frame giving rise to larger roll angles when cornering. This reduces stability and impacts on driver confidence.
The current state of the art for ADT front suspensions falls broadly into three groups. The most basic system comprises a beam axle mounted on a leading arm or frame pivotally connected to the front chassis frame on an axis parallel to the axle. This allows the wheels to move predominantly vertically but allows almost no suspension compliance in roll. Consequently this suspension can be fitted with very soft springs. A bump affecting both front wheels simultaneously can be negotiated with a high degree of comfort due to the soft springs. Single wheel bumps, however, are accompanied by a severe rolling action of the front chassis frame about the longitudinal axis of the articulation joint.
A second class of front suspension system comprises a beam axle mounted on a leading arm or frame pivotally connected to the front chassis frame by a ball joint with single or multiple control arms. The ball joint permits rolling of the front chassis relative to the front axle. The ability of the axle to roll relative to the chassis reduces the amount of lateral acceleration experienced by the driver when traversing single wheel bumps. However the suspension springs are now required to counter the tendency to roll when cornering. Increasing spring stiffness to counter excessive roll raises the suspension natural frequency and reduces suspension travel thus detracting from ride quality. Another disadvantage of this system is the large unsprung mass inherent in this design.
The third class of front suspension system used in state of the art ADTs has separate wheel carriers mounted on leading arms which are resiliently pivoted on the front chassis frame The leading arms have a rod and tube crosstie which keeps the wheels on the same axis while allowing rotation of one end of the crosstie with respect to the other. Combined with the compliance of the leading arm pivots, this allows the wheels a limited degree of articulation relative to each other in response to a single wheel bump.
In summary, none of the three presently used front suspension systems is able to offer drivers acceptable levels of both vertical and roll stability during operation at elevated forward speeds.
In the prior art GB1008045 (General Motors) discloses a dumper vehicle comprising a front and a rear chassis frame each supported on a pair of driven wheels and hinged together about a vertical steer axis located between the axes of rotation of the pairs of wheels and a tippable body which is pivoted on the rear frame and which, in its untipped position, extends over the steer axis, the centre of gravity of the body being located, in said untipped position, substantially mid-way between the said axes of rotation. The front and rear chassis frames only pivot about a single (vertical) axis for steering and thus this vehicle suffers from the rolling difficulties mentioned above.
As the vehicle has only four wheels the centre of gravity of the tippable body lies between the front and rear axles, substantially mid-way between the front and rear wheels putting considerable loading on the articulated connector.
GB1008045 (General Motors) further discloses such a dumper vehicle in which each pair of wheels is mounted on an axle connected to the associated chassis frame by a resilient suspension system so as to allow the axle to oscillate relative to the frame about a longitudinal axis. This appears to be the arrangement depicted in FIGS. 1 and 2, but it is not clear from the description how this is implemented. It also mentions the possibility that the wheels could be independently mounted on resilient suspension systems However this arrangement is not described and this proposal appears to have been incorporated without consideration of practical implementation. In practice, implementation of independent suspension would be difficult or impossible on a vehicle as shown in FIGS. 1 and 2 of GB1008045 because the large twin wheels shown, allow insufficient room to successfully install independent suspension.
U.S. Pat. No. 5,147,011 (Hvolka) describes a ‘uni-body’ digging service vehicle having an articulated chassis with a two-wheel front chassis frame plus four-wheel rear chassis frame. Here again, as with GB1008045, the articulation between front and rear sections occurs solely about a single vertical axis for steering purposes.
U.S. Patent Application Publication No. US2003/0094775 also discloses a forestry vehicle having a two-plus-four articulated chassis with articulation between front and rear segments only about a single vertical axis for steering purposes.
U.S. Pat. No. 3,414,072 (Hodges) discloses a highly maneuverable articulated vehicle, particularly for military use, comprising front and rear units each mounted on four wheels and interconnected by an articulated coupling. This construction facilitates travel over rough terrain as well as normal highway use. The driver and other personnel or cargo are carried in the rear unit.